Detection of and monitoring of material loss due to pitting, cracking and fractures, material conversion from corrosion and/or erosion, and material addition from material migration and accumulation, and material adsorption, as examples, in mechanical structures, and walls of pipes, vessels, and storage tanks in hard-to-access environments, such as under insulation or under paint, are important in many industries that involve liquid or gas storage and flow.
The detection of corrosion under insulation is done most effectively by visual inspection by removing the insulation, which is time consuming and costly. Other methods of detection include radiography, eddy current techniques, x-ray, remote TV monitoring, electromagnetic devices, local acoustic interrogation, and long-range acoustic interrogation using an array of acoustic transducers. These methods are not widely used because the information provided has too limited a range to be of practical value, or the apparatus involved is too cumbersome or expensive to implement. Moreover, existing methods for detecting anomalies in rigid structures are not capable of determining a particular location of the anomaly within a zone of interest.